In recent years, significcant development work has been done in the field of ink jet printing. One type of ink jet printing involves electrostatic pressure ink jet, wherein electrostatic ink is applied under pressure to a suitable nozzle. The ink is thus propelled from the nozzle in a stream which is caused to break up into a train of individual droplets which must be selectively charged and controllably deflected for recording, or to a gutter. A droplet formation may be controlled and synchronized by a number of different methods available in the art including physical vibration of the nozzle, pressure perturbations introduced into the ink supply at the nozzle, etc. The result of applying such perturbations to the ink jet is to cause the jet stream emerging from the nozzle to break into uniform droplets, often accompanied by smaller satellite droplets, at the perturbation frequency and at a predetermined distance from the tip of the nozzle. For some applied perturbations it is possible for drop formation without the formation of satellite droplets. It is of utmost necessity in such systems to precisely synchronize the application of the appropriate charging signal to the ink droplet stream at the precise time of droplet formation and break off from the stream. Means for supplying the selected electrostatic charge to each droplet produced by the nozzle conventionally comprises a suitable charging circuit and an electrode surrounding or adjacent to the ink stream at the location where the stream begins to form such droplets. Charging signals are applied between a point of contact with the ink and the charging electrode. A drop will thus assume a charge Q determined by the amplitude V of the particular signal on the charging electrode at the time the drop breaks away from the jet stream, and the capacitance C of the jet-charge electrode system, such that Q=CV. The capacitance C may be influenced by changes in the geometry at the tip of the jet stream. The drop thereafter passes through a fixed electric field and the amount of deflection is determined by the amplitude of the charge on the drop at the time it passes through the deflecting field. A recording surface is positioned down stream from the deflecting means such that the droplet strikes the recording surface and forms a small spot. The position of the drop on the writing surface is determined by the deflection that the drop experiences, which in turn is determined by the charge on the droplet. By suitably varying the charge, the location at which the droplet strikes the recording surface may be controlled with the result that a visible, human readable, printed record may be formed upon the recording surface. U.S. Pat. No. 3,596,275 of Richard G. Sweet, entitled "Fluid Droplet Recorder" discloses such a recording or printing system.
The time that the drop separates from the fluid stream emerging from the nozzle is quite critical since the charge carried by the droplet is produced at that moment by electrostatic induction. Accordingly, it is seen that the formation of satellite droplets produces an error in the charging sequence, and therefore produces a misregistration of droplets on the printing medium. The field established by the charging signal is maintaind during drop separation, and the drop will carry a charge determined by the instantaneous value of the signal at break off and by the geometric configuration of the tip of the jet at the time of droplet formation, which determines the jet-charge electrode capacitance C. In order to place exact predetermined charges on individual droplets in accordance with successive video signals, it is necessary to know exactly the time of drop break off in relationship to the timing of the charge signal and the shape of the droplet at break off. Stated differently, the droplet break off time and the application of the charge signal must be precisely synchronized. Failure to properly synchronize drop break off and the charging signal results in very imprecise control of the printing process with attendant degradation of the print quality. In addition, it is important to maintain a predetermined break off geometry in order to provide constant charging efficiency.
Synchronization may also be important in the binary type electrostatic printing wherein on-charge drops are not deflected and proceed directly to impact recording medium, whereas charge drops are deflected to the gutter. U.S. Pat. No. 3,373,437 of Richard G. Sweet et al., entitled "Fluid Droplet Recorder With a Plurality of Jets" discloses such a recording or printing system.
In this type of system if synchronization is not correct such that the charging signal is in the process of either rising or falling at the time of drop break off, the exact charge of the drop will be some time function of the maximum charge signal rather than being fully charged. Such drops may be deflected by an amount too small to cause impact with the gutter, but instead would impact the recording medium at an unintended position. With respect to the problem of obtaining proper synchronization between the charge signal and drop break off, the prior art definitely recognized the criticality of the synchronization problem and many techniques have been proposed to test the drops for proper charging and adjust the synchronization between the charging signals and the perturbation means. The following U.S. Pat. Nos. are representative of the prior art:
Lewis et al., 3,298,030; Keur et al., 3,465,350; Keur et al, 3,465,351; Lovelady et al., 3,596,276; Hill et al., 3,769,630 (above); Julisburger et al., 3,769,632 and Ghougasian et al., 3,836,912.
The Lewis et al. patent describes drop synchronization using a phase shifter to insure proper charging of drops at the correct time. The Keur et al., U.S. Pat. No. 3,465,350, describes the use of a test 33 KHz. train of slightly narrow pulses to charge drops for deflection to a test electrode, which is impacted only by fully charged drops. The detector thus supplies an output signal only when the phasing is correct. The Keur et al., U.S. Pat. No. 3,465,351 describes similar charging of the drops and the placement of a target bar so that all drops strike the bar, together with an integrated measurement of the total current given out by the drops to indicate proper or improper phasing. In both patents, the 33 KHz. charging rate for the test signals is the normal charging rate for the printing video signals. The Lovelady et al. patent also charges each drop of the stream to impact the gutter and directly compare the resultant gutter voltage against the reference voltage to establish whether the appropriate phase relationship exists. The Hill et al. patent discloses a dual gutter arrangement for using the voltage resulting from drops impacting at either extreme of deflection for detecting whether proper phasing has been achieved. The Julisburger et al. patent discloses the use of slightly narrow selective phase charging signals for testing the phase adjustment of each of a series of drops and an induction sensing means and digital phase detection circuitry for determining whether the drops are properly synchronized. The Ghougasian et al. patent is directed to a specific induction sensing means located near the charge electrode and prior to the deflection means useful for synchronization.
With the exception of the Keur et al., U.S. Pat. No. 3,465,350 and the Ghougasian et al. patents, all of the foregoing art is subjected to very poor signals and noise ratios on the detected signals and, as the result, is subject to a high probability of inaccuracy, or requires an intricatee array of shielding to attempt to reduce the signal to noise to usable levels. The Ghougasian et al. patent simply describes an induction sensor which may be utilized with the system of the Julisburger patent. The Keur et al. U.S. Pat. No. 3,465,350 is primarily an aiming test which may be effected by other parameters.
U.S. Pat. No. 3,969,733 of Richard A. DeMoss et al. which is assigned to the assignee of the present invention, teaches subharmonic charging and detection of charging phase synchronization in an ink jet system which employs electrostatic deflection of individual ink jet droplets. The phase control employs filtration/narrow-band amplification at a subharmonic frequency from the normal drop repetition frequency, such that noise and extraneous drop rate machine signals are filtered. Sensing is accomplished by an inductive charge sensing element operative with the gutter, and detection of the filtered sent signals by integration and by level detection is then provided to control circuitry for effecting the subsequent control of charging of the ink droplets.
Each of the above discussed prior art patent deals with drop formation efficiency. The drop formation efficiency is effected by the formation of droplets and accordingly is also effected by the formation of satellite droplets. This is so, since satellite droplets either merge in a forward or rearward direction, causing droplets of different size, and which arrive at the charging point at an incorrect time. Accordingly, spots on the recording medium are registered with different sizes, and at imprecise locations.
An article entitled "Investigation of Nonlinear Waves on Liquid Jets," appearing in The Physics of Fluids, Vol. 19, No. 8, August 1976 by Howard H. Taub describes the spectrum analysis of a liquid jet by the use of an optical probe. There is, however, no teaching of the use of the results of the spectrum analysis in a feedback control system to conrol the formation of satellite droplets in an ink jet printing system.
U.S. Pat. No. 3,928,855 of Helinski et al. discloses method and apparatus for controlling satellites in a magnetic ink jet printing system through the use of an asymmetrical perturbation. The asymmetrical excitation signal, such as a sawtooth wave, has substantial second and/or third harmonic content, which results in an excitation signal with different rise and fall times for producing an ink jet stream free of satellite droplets. There is, however, no teaching of providing the asymmetrical excitation signal as a function of a feedback control signal which results from sensing the surface profile of the ink jet stream prior to drop break off.
The existence of satellite droplets in an ink jet printing system is undesirable for the reasons set forth above. Two methods presently exist for satellite elimination, namely, utilizing a good head design which provides a good satellite print window, that is no satellites, by virtue of driving the piezoelectric transducer within a predetermined range of voltages, or utilizing harmonic injection.
A good head design would provide the best solution, however at the present time head design is inadequately understood and print windows are relatively unpredictable even when comparing two heads of ostensibly the same design. Moreover, elimination of satellites frequently require driving the piezoelectric driver quite hard with the result that the break off distance is shorter than desired when the whole head design is considered. For example, oftentimes there is inadequate space left for an airduct and charge electrode. This is particularly true for small nozzles having an orifice of 0.7 mls. or less. In addition, while it is usually possible to eliminate satellites, it is extremely difficult, if not impossible, to precisely control the droplet break off geometry.
Insertion of harmonics into the piezoelectric driver is a viable means for overcoming this problem, since appropriate harmonics can, in principle be injected to control the break off geometry for a predetermined drop rate. This technique however, is somewhat unstable with day-to-day and on-off operation and even over periods of hours with the head in continuous operation. This may result, for example, from the formation or movement of air bubbles in the head or from structural changes of the head due to temperature variation.
Also, in some systems, droplet characterics are determined in response to the sensing of droplets downstream from the charging electrode. Accordingly, the droplets then are effected by drop-to-drop retardation due to aerodynamic effects, as well as charge repulsion effects from droplet-to-droplet. At this downstream point, essentially all drop break off characteristics are lost.
Any variations in head geometry influence the efficiency at which the applied electrical perturbation drive signal is converted to a mechanical perturbation by the piezoelectric transducer on the ink jet manifold. Accordingly, the mechanical perburtation is influenced by different harmonic components of the drive signal in different ways. This in turn may result in a change in the drop formation geometry, which might give rise to a satellite droplet in a previously satellite-free condition, or more generally may correspond to a change in the shape of the droplet formed at the break off point. Since the charging efficiency of droplets breaking off within the charge electrode depends on the shape of the droplet at break off, the efficiency may be adversely affected.
The ideal time to sense the frequency, phase and amplitude components of the ink jet stream for determining drop break off characteristics is at the precise time droplets are formed therefrom. This is usually impossible to achieve, however, since the droplets are normally formed inside the charge electrode. Therefore, according to the present invention the drop break off characteristics are determined by sensing upstream of break off, rather than downstream as taught by the prior art. The continuous portion, that is the portion just prior to break off of the stream is sensed to determine the break off characteristics. In response to the sensed characteristics, a piezoelectric drive signal is provided which controls droplet formation, and accordingly provides increased drop charging efficiency.